Low noise amplifier and low noise amplifying method

Abstract

A low noise amplifier for operating in response to different gain modes is disclosed. The low noise amplifier includes a voltage adjusting circuit, which provides a first bias voltage at a first gain mode and provides a second bias voltage at a second gain mode, where the second bias voltage is different from the first bias voltage; and an amplifying circuit coupled to the voltage adjusting circuit, for providing a first transfer characteristic according to the first bias voltage during the first gain mode in order to amplify an input signal to generate an output signal, and for providing a second transfer characteristic according to the second bias voltage during the second gain mode in order to amplify the input signal to generate the output signal.

1. A low noise amplifier for operating in response to one of a first gain mode and a second gain mode, comprising:
 a voltage adjusting circuit, for providing a first bias voltage during the first gain mode and providing a second bias voltage during the second gain mode, where the second bias voltage is different from the first bias voltage; and 
 an amplifying circuit, coupled to the voltage adjusting circuit, for determining a first transfer characteristic according to the first bias voltage and thereby amplifying an input signal to generate an output signal according to the first transfer characteristic, and for determining a second transfer characteristic according to the second bias voltage and thereby amplifying the input signal to generate the output signal according to the second transfer characteristic, and the amplifying circuit comprising:
 a load; 
 a plurality of first transistors and at least one second transistor, wherein each of the first transistors and the second transistor has a first node coupled to the load for generating the output signal, a second node coupled to the input signal for receiving the input signal and a third node coupled to the voltage adjusting circuit for receiving the first bias voltage during the first gain mode and receiving the second bias voltage during the second gain mode; 
 a plurality of first switching devices, coupled between the first nodes of the first transistors and the load respectively, wherein each of the first switching devices controls its connected first transistor being coupled to or decoupled to the load; and 
 a controlling module, coupled to the first switching devices, for controlling an on/off state of each of the first switching devices with respect to the first gain mode and the second gain mode. 
 
 
   
   
     2. The low noise amplifier of  claim 1 , wherein the voltage adjusting circuit comprises:
 a controllable current source, for providing a first current during the first gain mode and providing a second current during the second gain mode, where the second current is different from the first current; and 
 a current to voltage converting module, coupled to the controllable current source, for converting the first current into the first bias voltage, and converting the second current into the second bias voltage. 
 
   
   
     3. The low noise amplifier of  claim 1 , wherein the amplifying circuit further comprises:
 a second switching device, coupled to the controlling module and coupled between the first node of the second transistor and a power source, for controlling the second transistor being coupled to or decoupled to the power source under the control of the controlling module. 
 
   
   
     4. The low noise amplifier of  claim 1 , wherein the amplifying circuit further comprises:
 a third transistor, coupled between the load and the first node of the second transistor, for controlling current flowing through the load. 
 
   
   
     5. The low noise amplifier of  claim 4 , wherein an aspect ratio of the third transistor is adjustable, and the current flowing through the load is in proportion to the aspect ratio of the third transistor. 
   
   
     6. The low noise amplifier of  claim 1 , wherein the voltage adjusting circuit comprises:
 a fixed current source for providing a reference current; and 
 a controllable current to voltage converting module, coupled to the fixed current source, for converting the reference current into the first bias voltage, and converting the reference current into the second bias voltage. 
 
   
   
     7. The low noise amplifier of  claim 6 , wherein the controllable current to voltage converting module is an adjustable resistance device. 
   
   
     8. The low noise amplifier of  claim 1 , wherein the voltage adjusting circuit is coupled to the amplifying circuit by utilizing a current mirror configuration. 
   
   
     9. The low noise amplifier of  claim 1 , wherein the amplifying circuit provides a first transconductance for the input signal during the first gain mode and a second transconductance for the input signal during the second gain mode, the first transconductance is substantially equal to the second transconductance. 
   
   
     10. The low noise amplifier of  claim 1 , wherein at least one of the first and second characteristics is linear.